1. Field of the Invention
This invention relates to a charge pump circuit.
2. Description of the Related Art
Systems in which an EEPROM (Electrically Erasable Programmable Read Only Memory) or a flash memory is programmed and erased and systems in which an LCD (Liquid Crystal Display) or an analog switch is driven require a voltage higher than a power supply voltage Vdd.
Embedded charge pump circuits are commonly used in such systems. The charge pump circuit is a simple circuit that is capable of boosting a voltage using a single power supply in the system so that a voltage higher than the power supply voltage is provided.
In a common charge pump circuit, charge transfer MOS transistors are connected in series to form multiple stages of pumping packets so that an input voltage is boosted. Assuming the input voltage is Vdd and a threshold voltage of a transistor is Vd, the boosted voltage Vout of an N-stage charge pump circuit is expressed by an equation below.Vout=(N+1)(Vdd−Vd)Further details may be found in Japanese Patent Publication No. 2001-211637, and in “On-chip High-Voltage Generation in NMOS Integrated Circuits Using an Improved Voltage Multiplier Technique,” IEEE Journal of Solid-State Circuit, SC-11, No. 3, pp. 374–378, June 1976.